Sound Velocities of Zoisite at High Pressures and Temperatures and Implications for the Water Content in Subducting Oceanic Crust

Abstract

The P- and S-wave velocities of zoisite were measured simultaneously using ultrasonic techniques under pressure and temperature of up to 11 GPa and 773 K, respectively. The results indicate that the velocity of zoisite increases with increasing pressure and decreases with increasing temperature. Using finite strain fitting, values of $K_{S0}=129\left(3\right)\text{GPa}$, $K_{S0}^{\prime }=4.9$(3), $G_0=73\left(2\right)\text{GPa}$, $G_0^{\prime }=2.0$(2), $G_0^{″}=-0.11\left(2\right){\text{GP}a}^{-1}$, ${\partial K}_S/\partial T=-0.019\left(4\right)\text{GPa}/K$, and $\partial G/\partial T=-0.010\left(4\right)\text{GPa}/K$ were obtained. The velocity structure in the region of the oceanic crust with a high velocity gradient was modeled. Our models imply that decomposition of zoisite and lawsonite into garnet may cause high velocity gradient in hot and cold subducting oceanic crusts, respectively. By combining seismological observations and the elastic properties of minerals, the average water contents in high velocity gradient regions in hot and cold oceanic crusts were estimated to be ∼2 wt% and ∼4 wt%, respectively.